Typical aircraft-mounted microwave antennas utilized, for instance, for detecting incoming missile radars, have in large part been configured as slot antennas within the wing or fuselage of an aircraft; or have involved so-called Vivaldi notch antennas used primarily for their ultra-wide bandwidth.
The problems with the slot antennas are, first and foremost, that the aircraft wing or fuselage must be specially configured or formed so as to house or carry the slot antenna. Oftentimes these antennas are spaced along the edge of the wing and the wing is provided with a so-called wing glove to protect the antennas from environmental erosion, including rain and particle erosion. The wing gloves are also utilized to maintain the appropriate airflow across the wing so as to eliminate turbulences which could be caused with an open slot.
Moreover, when Vivaldi notch antennas are utilized, at the higher frequencies these antennas are highly directional with a very narrow antenna lobe that in some cases precludes their use as an antenna to detect missiles coming in from all directions. While incoming missiles are provided in most instances with infrared seekers, they are first directed to the target aircraft through the utilization of microwave radar. It is therefore important to be able to detect an incoming missile from any direction and to provide sufficient countermeasure radiation to cause the missile to go off-target. It is also important that the antenna have a low radar cross-section, RCS, to avoid detection.
The microwave region of the electromagnetic spectrum is usually said to include 1 gigahertz frequencies up to 18 gigahertz, which requires an 18:1 frequency ratio of high frequency cutoff to low frequency cutoff. Slot antennas, on the other hand, usually have a 3:1 ratio and as a result, numbers of antennas are required tuned to adjacent bands so as to provide the required wideband coverage.
Moreover, Vivaldi notch antennas, while providing ultra-wide bandwidth due to the Vivaldi notch structure, are exceptionally directional. Moreover, they do not provide adequate gain across their entire bandwidth.
There is therefore a need for a robust low RCS ultra-wideband antenna having an omnidirectional radiation pattern in which the gain of the antenna is better than unity across the entire bandwidth. Not only are these antennas to be useful in surveillance, the antenna must also be useable in a transmit mode to provide a maximum amount of power on target. This in general voltage standing wave ratio, means that the VSWR of the antenna across its entire bandwidth must be less than 2:1.
Additionally, the antenna should be capable of handling high powers and should be able to handle at least 100-watt CW at the frequency of interest.
Such antennas are also required, for instance, for IFF purposes in which identification of friend or foe requires their use in a transponder-like environment. This means that the antenna must be ultra-wideband, have the same omnidirectional antenna characteristics as described above and must be relatively efficient across the entire bandwidth.
It is important that the antenna be as omnidirectional as possible and in general have a pattern associated with a monopole antenna and a ground plane.
By way of further background, if one utilizes a double cone or discone, the radiation pattern for these antennas is a dipole pattern which is not useful in detecting missiles coming up from directly beneath an aircraft because the missile will be in an antenna null. It is also important that, as is usual, one wants to look at the horizon and it is therefore important to have a major 360° lobe in the horizontal direction.
Note that U.S. Pat. No. 6,346,920 shows a broadband fan cone direction finding array in which the radiator has a partial cone shape. This type of antenna is not applicable to the above-mentioned applications and is for a different purpose altogether. Also, it will be appreciated that the major operating frequency of these antennas is between 200 MHz and 3 gigahertz, with the cones themselves being fabricated through the utilization of wires. Additionally, these cones are arrayed so as to provide direction finding capabilities in the VHF/UHF/SHF bands. As can be seen from this patent, both monocones and bicones are described as prior art in this patent. It is noted in this patent that when these conical antennas are arrayed, their radiation patterns tend to interfere with each other, which complicates direction finding processes.
U.S. Pat. No. 6,198,454 describes a similar fan cone direction finding antenna array, whereas U.S. Pat. No. 4,835,542 describes an ultra-wide band linearly polarized biconical antenna.
A biconical dipole antenna is described in U.S. Pat. No. 5,367,312, with the antenna being implemented through the use of wires distributed around a rod to define a conical cavity around each of the rods.
Finally, U.S. Pat. No. 5,068,671 describes an orthogonally polarized quadrophase electromagnetic radiator which has airfoil-shaped elements to define a horn and which has a ground plane member which is preferably a truncated conical shape.
None of these antennas describe a monocone over a ground plane, much less a way of providing an ultra-wideband response to a monocone, which also provides an omnidirectional pattern and high gain.